Flexible printed circuit boards (hereinafter, referred to as “FPC”) are often provided with a protective film (coverlay film) on the outer surface thereof for protection of the wiring (copper foil pattern and the like). Further, as an insulating protective film of flexible circuit boards and the like, polyimide resin, polyamideimide resin, polyurethane resin, epoxy resin and the like are employed. In recent years, there is a demand for a protective film having superior heat resistance and various protective films made of polyimide having superior heat resistance have been proposed. In such resin components, polysiloxane, polycarbonate, polybutadiene, polyester or the like is introduced as a flexible component, so that characteristics such as flexibility, bending resistance, low warping and the like are imparted to the resulting cured film.
In Patent Documents 1 and 2, compositions comprising a siloxane-modified polyimide derived from diaminopolysiloxane are disclosed. The siloxane-modified polyimide has excellent heat resistance and electric insulation property and is also excellent in low warping because of its low elasticity. Yet, there are also problems in that the surface of the cured film does not have sufficient adhesion due to the polysiloxane component and that cyclic siloxane is generated and released during high-temperature heating.
In Patent Document 3, a composition comprising a polycarbonate-modified polyamideimide resin is disclosed. This composition is excellent in low warping and the problems of insufficient adhesion of the cured film surface and generation of cyclic siloxane were improved therein. Yet, with the polyamideimide resin alone, the chemical resistance is still low; therefore, the composition needs to be thermally cured with an epoxy and there is a problem of storage stability due to addition of the epoxy. Further, the cured film also has a problem of moisture absorption due to the presence of amide bonds.
Patent Documents 4 and 5 disclose compositions comprising a polycarbonate-modified polyimide resin and Patent Document 6 discloses a composition comprising a polyimide resin modified with polycarbonate and polybutadiene. Thereamong, in Patent Document 4, a modified polyimide resin was obtained by reacting an isocyanate-terminated polycarbonate, aromatic isocyanate and tetracarboxylic dianhydride. However, there are problems in that the types of available aromatic isocyanates are limited and that the chemical resistance of the resin tends to be lowered when the resin is solubilized, so that it is still required to impart with thermosetting property to the resin using an epoxy. In Patent Document 5, in order to easily obtain a soluble modified polyimide, a modified polyimide was prepared by reacting a polycarbonate diol, hydroxyl group-terminated imide oligomer and diisocyanate compound. However, because of the problem in the solubility of the hydroxyl group-terminated imide oligomer, practically, the structure between the hydroxyl group and imide group must be an aliphatic structure, so that the heat resistance tends to be low. Further, the resin composition disclosed in Patent Document 6 is also a thermosetting type as in the case of those disclosed in Patent Documents 4 and 5.